1. Field of the Invention
The present invention is in the field of window structures and more particularly, multi-pane insulative window structure, having an electrically heated pane.
2. Description of the Prior Art
It has long been known that an insulative window structure can be formed by mounting two or more panes of transparent material in a spaced-apart relationship so that an insulative layer of dead air is contained between the panes. It is also known that when such an insulative window structure is used to separate a cold region from a warmer region there is a tendency for condensation or frost to form on the exposed surface of the pane adjacent the warm region. The frost detracts from the utility and aesthetics of the structure. This tendency results from the fact that the pane adjacent the warmer region is usually at a cooler temperature than the air in the warmer region due to radiative and conductive heat loss from the pane.
To overcome this tendency for condensation and frost to form on the pane adjacent the warmer region, it is known in the art to provide electrical heating elements to heat the pane adjacent the warmer region. The heating elements may take various forms such as resistive wire distributed over the surface of the pane, or a conductive coating uniformly distributed on the surface. The conductive coating is desirably transparent to visible radiation, and usually is disposed on the unexposed surface of the pane so that the conductive surface is protected by the pane.
Of the heat thus supplied electrically to the pane adjacent the warmer region, a large portion is radiated to the adjacent pane which in turn normally radiates to the next colder pane or to the colder region. In most applications, it is uneconomical to heat more than one pane of the multi-pane structure. It is normally equally uneconomical to heat the colder region adjacent the window structure, particularly where energy is required to remove heat from the cooler region as, for example, in refrigerators and freezers.
The use of an electrically conductive element for heating a mirror is shown in Van Laethem, et al., U.S. Pat. No. 3,790,748. The mirror coating used by Van Laethem is not an infrared reflecting light transmitting coating. Further, a layer of electrically insulating material is included between the reflecting coating and the electrically conductive coating in the various embodiments of Van Laethem, et al.
In U.S. Pat. No. 3,612,825, Chase shows the use of the grid-like or foraminous coating applied to an inner pane of a three pane oven window which reflects radiant heat back into the oven to retain the oven heat and to keep the outer pane at a cooler temperature. The Chase oven window makes use of a convection of ambient air between the outermost and the middle pane to carry heat away from the outer window into the cooler region outside the oven.
The problem addressed by the present invention is different in that windows in a refrigerator door structure typically and desirably occupy an appreciable fraction of the area of the refrigerated cabinet front. This large expanse of window must remain clear of moisture and frost so as to display the contents of the refrigerated cabinet in an aesthetically pleasing manner at all times.
It is thus normally necessary to maintain the temperature of the outer pane above the dew point temperature of the surrounding air. Generally, this requires that the outer pane be heated. This approach, although widely used, imposes an appreciable and expensive heat load on the refrigeration system because the heated outer pane radiates readily into the colder interior of the refrigerator.
Thus, a dilemma results. If the outer pane is not heated, the formation of moisture and frost seriously detract from the cabinet's utility for displaying merchandise. On the other hand, if the outer pane is heated, the heat generated renders the unit less efficient and more expensive as a refrigerator. This is the unique problem for which the present invention provides a novel solution.